Fluidized bed coking (fluid coking) is a petroleum refining process in which heavy petroleum feeds, typically the non-distillable residue (resid) from fractionation or heavy oils are converted to lighter, more useful products by thermal decomposition (coking) at elevated reaction temperatures, typically about 480 to 590° C., (about 900 to 1100° F.) and in most cases from 500 to 550° C. (about 930 to 1020° F.). Heavy oils that may be processed by the fluid coking process include heavy atmospheric resids, vacuum resids, aromatic extracts, asphalts, and bitumen from oil sands.
The process is carried out in a unit with a large reactor vessel containing hot coke particles that are maintained in the fluidized condition at the required reaction temperature with a fluidizing gas (e.g., steam) injected at the bottom of the vessel. The heavy oil feed (e.g., bitumen) is heated to a pumpable temperature, typically in the range of 345 to 400° C. (about 660 to 750° F.), mixed with atomizing steam, and fed through multiple feed nozzles arranged at several successive levels in the reactor. Steam is injected into a stripper section at the bottom of the reactor and passes upwards through the coke particles in the stripper as they descend from the main part of the reactor above. The feed liquid coats the coke particles in the fluidized bed, which make up the emulsion phase of the fluidized bed. As the thermal cracking reactions proceed, the liquid is transformed to vapour, which must migrate from the emulsion phase into the bubble phase in order to exit the system. The hydrocarbon vapours produced from the feed also aid in maintaining the fluidized condition. A layer of coke is formed and deposited on the particles, increasing the average particle size in the fluidized bed. In addition, agglomerates are often formed when several coke particles stick together. In order to maintain the particle size in the fluidized bed, high velocity steam is injected through multiple attrition nozzles that are located just above the stripper section. These high velocity steam jets grind particles together, reducing the size of large particles and agglomerates.
Liquid yields in fluid coking can be increased by reducing the reaction severity, or the time that molecules are exposed to process temperature. The typical approach taken to reduce reactor severity is to reduce reactor temperature. However, the downside of reducing temperature is increased stripper and sore thumb fouling, which can lead to reduced run lengths. Further, reducing the temperature can cause higher levels of wall coke to form, which, again, reduces the run length. Another approach to reduce reactor severity is to decrease the exposure time at high temperatures by providing short vapour phase residence times.
Long hydrocarbon vapour residence times are the most likely contributor to higher than expected “gas make”, defined as C4-components, in the fluid coking process. Suppression of feed liquid vapourization, coupled with less than adequate mass transfer between the emulsion and bubble phases, is the most probable mechanism responsible for high “coke make”, defined as the toluene insoluble solid by-product of the thermal cracking reaction. Both phenomena result in lower liquid yields, and preliminary estimates suggest that they can contribute to as much as 11 wt % liquid yield loss. Optimizing the rate of removal of vapour from the emulsion phase should reduce the overall hydrocarbon vapour residence time of the reactor, increase liquid yields, and reduce gas make and coke make. It is estimated that a 3-5 wt % liquid yield increase can be achieved through maximizing vapour recovery from the reactor dense bed.